In the art, there are usually two methods for converting an AC voltage to a DC voltage.
According to the first method, a diode bridge circuit and a smoothing capacitor are used. Here, the diode bridge circuit carries out full-wave rectification of the AC power from an AC power supply. The smoothing capacitor smoothes the DC power output after the full-wave rectification.
According to the first method, in any state of the AC voltage, either positive or negative, a current always flows in a series circuit including two diodes. In this case, in the two diodes, a power loss is incurred corresponding to the product of the current flowing in each of the diodes and the forward voltage on each of the diodes.
According to the second method, a power factor improving converter (PFC) lies between the diode bridge circuit and the smoothing capacitor used in the first configuration. Here, the power factor improving converter boosts the DC voltage obtained by the full-wave rectification with the diode bridge circuit.
In the second method, during the full-wave rectification, since current flows in the serial circuit including two diodes, a power loss takes place. In addition, as a current alternately flows in a field effect transistor (FET) and a diode that constitute the power factor improving converter, a further power loss takes place.
In addition, for the power factor improving converter, since the waveform of the input current has to be a sinusoidal wave, the output voltage has to be set higher than the input voltage. However, the voltage needed by the load may not necessarily be a voltage higher than the input voltage. In such a case, a voltage-decreasing converter is connected to a subsequent stage of the power factor improving converter, so that the voltage boosted by the power factor improving converter is decreased to a desired voltage. However, a power or energy loss also occurs in this voltage decreasing operation. The overall power conversion apparatus is constituted by three sections, namely, an AC-DC conversion section, a DC-DC conversion (voltage boosting) section, and a DC-DC conversion (voltage decreasing) section. The power loss is the product of the loss of each. For example, when the efficiency of each section is 0.95, the total efficiency for the three sections becomes 0.95×0.95×0.95=0.86. That is, even when each section has a conversion efficiency as high as 95%, the 3-section configuration has an overall efficiency of only 86%. In this way, although each individual section may have a high conversion efficiency, the overall conversion efficiency of a multi-stage configuration becomes much lower.
There is a high demand for the development of a scheme that can conserve energy for electronic equipment. As a part of this scheme, higher conversion efficiency has been pursued for the power conversion apparatus that supplies power to the load. However, the conventional circuit configurations have only limited effect in improving conversion efficiency.